1. Field of the Invention
The invention relates to a catalyst degradation detection apparatus.
2. Description of the Related Art
In internal combustion engines mounted in vehicles, such as motor vehicles and the like, an exhaust passageway is provided with a catalyst for exhaust emission control, whereby NOx, HCs and CO in exhaust gas that flows in the exhaust passageway are removed. Besides, in order to remove these three components of exhaust gas, the catalyst is equipped with an oxygen storage function, and stoichiometric air/fuel ratio control of controlling the air/fuel ratio of the air-fuel mixture in a combustion chamber of the internal combustion engine to the stoichiometric air/fuel ratio is performed.
The oxygen storage function of the catalyst herein means a function of storing oxygen from exhaust gas into the catalyst according to the oxygen concentration in exhaust gas that passes through the catalyst, and of desorbing oxygen stored in the catalyst and releasing it from the catalyst into exhaust gas. Specifically, during a state in which the oxygen concentration in exhaust gas is higher than a value of oxygen concentration obtained when a mixture whose air/fuel ratio is set at the stoichiometric air/fuel ratio is burned in the combustion chamber, that is, during a state in which a mixture whose air/fuel ratio is leaner than the stoichiometric air/fuel ratio is burned in the combustion chamber, oxygen in the exhaust gas that passes through the catalyst is stored into the catalyst due to the foregoing oxygen storage function of the catalyst. On the other hand, during a state in which the oxygen concentration in exhaust gas is lower than the value of oxygen concentration obtained when the mixture whose air/fuel ratio is set at the stoichiometric air/fuel ratio is burned in the combustion chamber, that is, during a state in which a mixture whose air/fuel ratio is richer than the stoichiometric air/fuel ratio is burned in the combustion chamber, oxygen stored in the catalyst is desorbed and released from the catalyst into the exhaust gas due to the oxygen storage function of the catalyst.
Besides, in the foregoing stoichiometric air/fuel ratio control, the amount of fuel injection of the internal combustion engine is adjusted according to the oxygen concentration in exhaust gas so that the oxygen concentration in exhaust gas becomes equal to the value of oxygen concentration obtained when the mixture whose air/fuel ratio is set at the stoichiometric air/fuel ratio is burned in the combustion chamber. This stoichiometric air/fuel ratio control uses a pre-catalyst sensor that is provided in the exhaust passageway upstream of the catalyst and that outputs a signal based on the oxygen concentration in exhaust gas, and a post-catalyst sensor that is provided in the exhaust passageway downstream of the catalyst and that outputs a signal based on the oxygen concentration.
Specifically, the amount of fuel injection of the internal combustion engine is adjusted according to the signal from the pre-catalyst sensor so that the oxygen concentration in exhaust gas upstream of the catalyst becomes equal to the value of oxygen concentration obtained when the mixture whose air/fuel ratio is set at the stoichiometric air/fuel ratio is burned in the combustion chamber. Due to this adjustment, the air/fuel ratio of the mixture in the combustion chamber of the internal combustion engine is controlled so as to converge to the stoichiometric air/fuel ratio while fluctuating between the fuel-rich side and the fuel-lean side of the stoichiometric air/fuel ratio. However, in the case where only the adjustment of the fuel injection amount commensurate with the output signal of the pre-catalyst sensor is performed, there is still a possibility that the center of fluctuation of the air/fuel ratio of the internal combustion engine that fluctuates between the rich and lean sides so as to converge to the stoichiometric air/fuel ratio as described above will deviate from the stoichiometric air/fuel ratio, due to product variations of the pre-catalyst sensor, or the like. In order to correct such a deviation, the fuel injection amount of the internal combustion engine is adjusted also according to the signal output by the post-catalyst sensor so that the center of fluctuation of the air/fuel ratio of the internal combustion engine that fluctuates between the rich side and the lean side due to the adjustment of the fuel injection amount commensurate with the signal from the pre-catalyst sensor becomes equal to the stoichiometric air/fuel ratio.
Thus, by equipping the catalyst with the oxygen storage function and performing the stoichiometric air/fuel ratio control, it becomes possible to effectively remove the three components in exhaust gas, that is, NOx, HCs and CO. Specifically, when, during execution of the stoichiometric air/fuel ratio control, the air/fuel ratio of the mixture in the combustion chamber changes to the lean side, the oxygen concentration in exhaust gas that passes through the catalyst becomes higher than the value of oxygen concentration obtained when the mixture whose air/fuel ratio is set at the stoichiometric air/fuel ratio is burned in the combustion chamber, so that oxygen in the exhaust gas that passes through the catalyst is stored into the catalyst and therefore NOx in the exhaust gas is reduced. On the other hand, when, during execution of the stoichiometric air/fuel ratio control, the air/fuel ratio of the mixture in the combustion chamber changes to the rich side, the oxygen concentration in exhaust gas that passes through the catalyst becomes lower than the value of oxygen concentration obtained when the mixture whose air/fuel ratio is set at the stoichiometric air/fuel ratio is burned in the combustion chamber, so that oxygen stored in the catalyst desorbs from the catalyst, and oxidizes HCs and CO in the exhaust gas. Therefore, during execution of the stoichiometric air/fuel ratio control, when the air/fuel ratio of the mixture in the combustion chamber fluctuates between the rich and lean sides as the air/fuel ratio converges to the stoichiometric air/fuel ratio, the three components of exhaust gas, that is, NOx, HCs and CO, are effectively removed.
Incidentally, as for the catalyst, the oxygen storage function declines as the catalyst degrades. Therefore, it has been proposed to find a maximum value of the amount of oxygen stored in the catalyst (hereinafter, termed the oxygen storage amount), and to determine, on the basis of the oxygen storage amount, whether or not degradation of the degrade catalyst is present. For example, in Japanese Patent Application Publication No. 2008-31901 (JP-A-2008-31901), the presence or absence of degradation of the catalyst is determined by the following procedure.
When the air/fuel ratio of the mixture burned in the combustion chamber of an internal combustion engine is forced to change between the rich and lean sides as shown in a time chart of the air/fuel ratio in FIG. 10 (at a timing ta), a change occurs in the signal of a pre-catalyst sensor correspondingly as shown in a time chart of the pre-catalyst sensor's output in FIG. 10. Incidentally, a timing tb in the time chart of the pre-catalyst sensor's output in FIG. 10 is a timing at which the signal of the pre-catalyst sensor comes to have a value that corresponds to the oxygen concentration in exhaust gas that results when the mixture at the stoichiometric air/fuel ratio is burned. Then, the amount of oxygen stored into the catalyst or desorbed from the catalyst during a period (tb to td) from when the foregoing change occurs in the signal of the pre-catalyst sensor till when a change that corresponds to the change in the air/fuel ratio occurs in the signal of the post-catalyst sensor is calculated. Incidentally, the determination that a change that corresponds to the change in the air/fuel ratio has occurred in the signal of the post-catalyst sensor can be made on condition that the signal has reached a criterion value H set for making the determination as shown by a solid line in the time chart of the post-catalyst sensor's output 1 in FIG. 10.
For example, if the forced change in the air/fuel ratio occurs from the rich side to the lean side, oxygen is stored into the catalyst during the period (tb to td). Then, the amount of oxygen stored into the catalyst during the period is calculated, and the calculated amount of oxygen is determined as the oxygen storage amount of the catalyst. Incidentally, the oxygen storage amount thus found changes during the period (tb to td) as shown by a solid line in a time chart of the oxygen storage amount 1 in FIG. 10. On the other hand, if the forced change in the air/fuel ratio occurs from the lean side to the rich side, oxygen is desorbed from the catalyst during the period (tb to td). Then, the amount of oxygen desorbed from the catalyst during the period is calculated, and the calculated amount of oxygen is determined as the oxygen storage amount of the catalyst. Incidentally, the oxygen storage amount thus found also changes during the period (tb to td) as shown by the solid line in the time chart of the oxygen storage amount 1 in FIG. 10.
Then, in order to determine the presence or absence of degradation of the catalyst, the oxygen storage amount found at the time point of the end of the period (tb to td) with a threshold value set for the determination regarding the degradation. Concretely, if the oxygen storage amount is less than the threshold value, it can be determined that decline of the oxygen storage function due to degradation of the catalyst has occurred, and therefore it is determined that degradation of the catalyst is present. On the other hand, if the oxygen storage amount is greater than or equal to the threshold value, it can be determined that decline of the oxygen storage function due to degradation of the catalyst has not occurred, and therefore it is determined that degradation of the catalyst is not present (the catalyst is normal).
However, in the foregoing determination as to the presence or absence of degradation of the catalyst, it sometimes happens that the oxygen storage amount found for use for the determination deviates from a proper value to the increase side due to the effect of deterioration of the responsiveness of the signal from the post-catalyst sensor to a change in the oxygen concentration in exhaust gas downstream of the catalyst. For example, if the deterioration of the responsiveness of the post-catalyst sensor appears in the signal of the same sensor in the form of a change from the transition shown by the solid line in the time chart of the post-catalyst sensor's output 1 in FIG. 10 to the transition shown by a two-dot chain line in the same time chart, the period for which the oxygen storage amount is calculated increases from the period from tb to td to a period from tb to tf. In consequence, the oxygen storage amount found at the end time point of the period (tb to tf) is a value (value at the timing tf) that is excessively larger than a proper value (value at the timing td), as shown by the two-dot chain line in the time chart of the oxygen storage amount 1 in FIG. 10. Then, if the presence or absence of degradation of the catalyst is determined on the basis of the oxygen storage amount whose value is deviated from the proper value to the increase side, an error may sometimes occur in the determination.
As a countermeasure against the foregoing problem, JP-A-2008-31901 discloses that a travel distance of the vehicle or an accumulated operation time of the internal combustion engine is measured as a parameter which correlates with the responsiveness of the post-catalyst sensor, and that the criterion value H is corrected on the basis of the measured parameter (which corresponds to the responsiveness of the post-catalyst sensor). Specifically, the criterion value H is corrected so as to make the determination more gentle (so as to be positioned higher in the time chart of the post-catalyst sensor's output 1 in FIG. 10) the more the parameter comes to deteriorate the responsiveness of the post-catalyst sensor. In this case, the criterion value H is corrected on the basis of the parameter (the responsiveness of the post-catalyst sensor) so that the two-dot chain line L1 in this time chart reaches the post-correction criterion value H at the timing td. In this manner, the oxygen storage amount that is found is restrained from deviating from a proper value to the increase side due to deterioration of the responsiveness of the post-catalyst sensor.
As described above, by correcting the criterion value H on the basis of the parameter that correlates with the responsiveness of the post-catalyst sensor, the restraint of deviation of the oxygen storage amount from a proper value due to deterioration of the responsiveness of the post-catalyst sensor can be pursued.
However, the effect of deterioration of the responsiveness of the post-catalyst sensor does not necessarily appear in the signal from the post-catalyst sensor in a manner as shown by the two-dot chain line in the time chart of the post-catalyst sensor output 1 in FIG. 10, and may possibly appear in the signal from the post-catalyst sensor in a manner that is different from the manner shown by the two-dot chain line, depending on the state of operation of the internal combustion engine, or the like. For example, even when the degree of deterioration of the responsiveness of the post-catalyst sensor is substantially equal to the degree of deterioration shown by the two-dot chain line in the time chart of the post-catalyst sensor output 1 in FIG. 10, the effect of deterioration of the responsiveness of the post-catalyst sensor may possibly appear in the signal of the post-catalyst sensor, for example, in a manner as shown by a two-dot chain line L2 in a time chart of the post-catalyst sensor's output 2 in FIG. 10 or a two-dot chain line L3 in a time chart of the post-catalyst sensor's output 3 in FIG. 10, depending on the state of operation of the internal combustion engine, or the like. Incidentally, the two-dot chain line L1 in each of the time chart of the post-catalyst sensor's output 2 and the time chart of the post-catalyst sensor's output 3 is the same as a two-dot chain line in the time chart of the post-catalyst sensor's output 1 in FIG. 10.
As can be seen from the time chart of the post-catalyst sensor's output 2 in FIG. 10, the two-dot chain line L2 is positioned below the two-dot chain line L1 before the timing tf, and coincides with the two-dot chain line L2 after the timing tf. In this case, the two-dot chain line L2 reaches the post-correction criterion value H at a timing tc prior to the timing td. Therefore, at the timing tc, the oxygen storage amount in the period from tb to tc is found. Incidentally, during this period, the oxygen storage amount changes as shown in a time chart of the oxygen storage amount 2 in FIG. 10. The oxygen storage amount found at the timing tc is a value deviated from a proper value (a value on the solid line at the timing td in the time chart of the oxygen storage amount 1 in FIG. 10) to the decrease side. Therefore, if the presence or absence of degradation of the catalyst is determined on the basis of the oxygen storage amount that is found at the timing tc, an error may sometimes occur in the determination.
On another hand, as can be seen from the time chart of the post-catalyst sensor's output 3, the two-dot chain line L3 is positioned above the two-dot chain line L1 before the timing tf, and coincides with the two-dot chain line L1 after the timing tf. In this case, the two-dot chain line L3 reaches the post-correction criterion value H at a timing te after the timing td. Therefore, at the timing te, the oxygen storage amount during the period from tb to te is found. Incidentally, during this period, the oxygen storage amount changes as shown in the time chart of the post-catalyst sensor's output 3 in FIG. 10. The oxygen storage amount found at the timing te is a value deviated from a proper value (the value on the solid line at the timing td in the time chart of the oxygen storage amount 1 in FIG. 10) to the increase side. Therefore, if the presence or absence of degradation of the catalyst is determined on the basis of the oxygen storage amount that is found at the timing te, an error may sometimes occur in the determination.
As described above, if the effect of deterioration of the responsiveness of the post-catalyst sensor which appears in the signal from the post-catalyst sensor varies as shown by the two-dot chain line L2 in the time chart of the post-catalyst sensor's output 2 in FIG. 10 and the two-dot chain line. L3 in the time chart of the post-catalyst sensor's output 3 in FIG. 10 with reference to the two-dot chain line L1 in the time charts of the post-catalyst sensor's output 2 and the post-catalyst sensor's output 3 in FIG. 10, the oxygen storage amount for use for the determination as to the presence or absence of degradation of the catalyst is deviated from the proper value as described above. Since there is possibility that the presence or absence of degradation of the catalyst may be determined on the basis of the oxygen storage amount deviated from the proper value, it cannot be clearly said that a result of the determination is definitely proper.